There is a need in many industrial fields for hollow bodies without any macroscopic surface discontinuity. As a rule, the function of such bodies is to lessen the weight of a part while enabling it to meet the requirements of the particular application. These hollow spheres in particular may be used to make a modular composite material characterized essentially by its low weight and its isotropic properties easily fitting the particular needs. Hollow spheres also find significant applications in catalytic materials because they provide very large specific surfaces per unit weight. Furthermore there are more conventional applications for hollow bodies, in particular in mechanical engineering: balls of ball-bearings, hollow mechanical parts offering very low weight and suitable mechanical strength, and the like.
Presently several kinds of processes are known for making hollow bodies, in particular hollow balls or spheres. In all these processes, the hollow bodies are manufactured sequentially and undergo individual fabrication stages requiring accurate positioning of each piece. Consequently these procedures are costly as a rule while automating them demands complex and costly equipment.
A first type of procedure comprises making two shells by molding or stamping and in assembling them by any known means. This procedure comprises several sequential stages each requiring accurate positioning on work sites and its use is suited only for high-value units.
Another type of procedure employed in particular in the manufacture of small balls for small chains comprises stamping them from a tube. While much more economical, this procedure however does incur several drawbacks: in the first place, it does not provide a continuous ball surface because each ball will comprise two holes; further, the stamping technique used only allows making balls over a narrow range of thickness and only with a very restricted selection of materials, namely materials capable of flow without cracking.
Another procedure offers the advantage of accurately reproducing a given shape and comprises making each hollow body individually by electroforming at the end of a soluble electrode around an expendable mandril. However by its very nature this is a very costly procedure which furthermore results in hollow bodies including a removal orifice.
Another type of procedure comprises coating a core with a solid and then fashioning a hole in this coating to let through a solvent for dissolving the core (French patent No. 1,311,777; U.S. Pat. No. 4,464,231). However this procedure requiring individual mechanical drilling of each ball is incompatible with mass production and therefore incurs the same disadvantages as above. Moreover, the hole in the ball degrades its homogeneity and its overall strength and resistance.
For completeness, mention also must be the very ancient procedure of glass blowing which however is restricted to this material and causes difficulties in controlling the shape of the hollow body.
The primary object, then, of this invention is to provide a novel manufacturing method for continuous, closed and hollow bodies.
Another object of the invention is to provide a method which can be implemented on pieces in bulk which therefore need not be positioned during the processing stages.
Still another object is to enable the manufacture of hollow bodies, in particular hollow spheres each comprising a continuous skin free from any macroscopic perforation.
Yet another object is to enable the manufacture of hollow bodies of diverse materials and with thickness which can be easily adjusted in relation to the properties sought.
A further object is to create composite hollow bodies, that is bodies of which the skin comprises several layers which may have different properties and which may be combined in order to meet the requirements of the particular application.
Yet a further object is to provide a method allowing the economical manufacture and in very large quantities of small hollow balls with an outer diameter larger than 0.6 mm and a skin thickness at least 50 microns.
Still another object is allow matching the surface condition of the hollow bodies or balls to the applications under consideration.